Probing large viscosities in glass-formers with nonequilibrium simulations

Significance As a liquid cools, molecules move more slowly and the viscosity rises. A fundamental question is whether this trend continues smoothly down to zero temperature, or if flow stops at a finite temperature where the material undergoes a transition to a glass phase. Direct measurements of growing viscosities become difficult as the time for motion exceeds years or centuries. We describe and test an approach for obtaining large viscosities using nonequilibrium molecular dynamics simulations. Results agree with existing experiments on the model glass-former squalane and allow viscosities over 10 orders of magnitude larger to be predicted. The temperature dependence at fixed pressure or density is consistent with a gradual slowing of dynamics, rather than a finite-temperature divergence in viscosity. For decades, scientists have debated whether supercooled liquids stop flowing below a glass transition temperature Tg0 or whether motion continues to slow gradually down to zero temperature. Answering this question is challenging because human time scales set a limit on the largest measurable viscosity, and available data are equally well fit to models with opposite conclusions. Here, we use short simulations to determine the nonequilibrium shear response of a typical glass-former, squalane. Fits of the data to an Eyring model allow us to extrapolate predictions for the equilibrium Newtonian viscosity ηN over a range of pressures and temperatures that change ηN by 25 orders of magnitude. The results agree with the unusually large set of equilibrium and nonequilibrium experiments on squalane and extend them to higher ηN. Studies at different pressures and temperatures are inconsistent with a diverging viscosity at finite temperature. At all pressures, the predicted viscosity becomes Arrhenius with a single temperature-independent activation barrier at low temperatures and high viscosities (ηN>103 Pa⋅s). Possible experimental tests of our results are outlined.

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